I couldn't find anything about checking serial numbers on the Peavey site.
How do I figure out how old my 5150 II is?

Check out my band! Endeavor

Fender American Deluxe Telecaster (little 59')
Peavey 5150 II
Orange PPC412
Boss NS-2
Boss DD-3
Monster Power Pro 900
Korg Pitchblack Pro
Sennheiser EW 172 Wireless
Carvin 6 space rack
Obviously you cut it in half and count the rings!

Unfortunately, I have no knowledge on the subject. Why not contact Peavey?
I let you get away so cleanly.